Lithography is used in the fabrication of semiconductor devices. In lithography, a light-sensitive material, called a “photoresist,” coats a wafer substrate, such as silicon. The photoresist may be exposed to light reflected from a mask to reproduce an image of the mask that is used to define a pattern on the wafer. When the wafer and mask are illuminated, the photoresist undergoes chemical reactions and is then developed to produce a replicated pattern of the mask on the wafer.
Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength in a range of from about 11 nm to about 15 nm, e.g., 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere, it will be understood that the term “light” is used to encompass electromagnetic radiation regardless of whether it is within the visible part of the spectrum.
EUV light may be produced using a small, hot plasma which will efficiently radiate at the desired wavelength. The plasma may be created in a vacuum chamber, typically by driving a pulsed electrical discharge through the target material (discharge produced plasma or “DPP”), or by focusing a pulsed laser beam onto the target material (laser produced plasma or “LPP”). The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV part of the spectrum. The light produced by the plasma is then collected by nearby EUV optics such as mirrors and sent downstream to the rest of the lithography tool.
The hot plasma tends to erode materials nearby, e.g., the electrodes in electric-discharge sources or components of the gas delivery system in laser-produced plasmas. The eroded material may coat the EUV optics, resulting in a loss of reflectivity and reducing the amount of light available for lithography. Also, debris in the form of unvaporized target material can contaminate the surfaces of the EUV optics. It then becomes necessary to clean the surface of the EUV optic. One known technique for cleaning an EUV optic is to use a plasma generated with high frequency RF electric field, i.e., an RF plasma. The actual implementation of plasma cleaning, however, presents major technical challenges. Space constraints of a real LPP source make it very difficult to implement plasma cleaning without negatively affecting other source functions such as by causing undesirable reduction of the EUV collection angle or debris scattering from new components introduced to create the RF plasma.
With the above in mind, applicant discloses systems and methods for cleaning optics in a laser produced plasma EUV light source.